In the specification of U.S. Pat. No. 4,517,803, a turbocharger compressor is described for delivering air under pressure to the induction system of an engine. The compressor has a housing which accommodates a controllable diaphragm valve permitting selective recirculation of air via a recirculation duct from a compressor delivery to the compressor inlet to avoid surging the compressor during engine transients when the throttle is closed.
While such a valve arrangement is inexpensive and functionally satisfactory, it can introduce undesirable noise during the transition between open and closed conditions and the object of the present invention is to improve such a compressor by reducing the discernible noise produced by the controllable valve.